1. Field of the Invention
The present invention relates to a liquid crystal display device used for display sections of computers and OA apparatuses and the like. More specifically, the present invention relates to a liquid crystal display device which has excellent display characteristics and a high aperture ratio.
2. Description of the Related Art
Conventionally, a liquid crystal display device using an active matrix substrate is known as a display device for computers and OA apparatuses. An example of such a liquid crystal display device using an active matrix substrate is shown in FIG. 26. The active matrix substrate in this example has thin film transistors (hereinbelow referred to as TFTs) as switching elements.
Referring to FIG. 26, TFTs 106 and pixel capacitors 108 are formed in a matrix on a substrate made of glass or the like. A gate electrode of each TFT 106 is connected to a corresponding gate signal line 104, so that the TFT 106 is switched on and off in response to a signal input into the gate electrode via the gate signal line 104. A source electrode of the TFT 106 is connected to a corresponding source signal line 102, so that a video signal is input into the TFT 106. A drain electrode of the TFT 106 is connected to a pixel electrode and one terminal of the corresponding pixel capacitor 108. The other terminal of the pixel capacitor 108 is connected to a corresponding pixel capacitor line 110 and also connected to a counter electrode provided on a substrate facing the active matrix substrate.
FIG. 27 is a plan view of such an active matrix substrate, FIG. 28 is a sectional view taken along line 28xe2x80x9428 of FIG. 27, and FIG. 29 is a sectional view taken along line 29xe2x80x9429 of FIG. 27.
Referring to FIGS. 27 and 28, each pixel of the liquid crystal display device includes the TFT 106 (see FIG. 26), an extended drain electrode 125, a storage capacitor electrode 126, and a pixel electrode 140. Referring to FIG. 29, for each pixel, the gate signal line 104 together with the gate electrode, a gate insulating film 103, a semiconductor layer 134, a channel protection layer 128, an n+-Si layer 130 and an ITO (indium tin oxide) film 132 which together constitute the source and drain electrodes, the source signal line 102 made of a metal layer, an interlayer insulating film 136, and the pixel electrode 140 made of a transparent conductive layer are formed in this order on a transparent insulating substrate 120, to form the active matrix substrate. The pixel electrode 140 is connected to the drain electrode of the TFT 106 via a contact hole 142 (see FIG. 28) formed through the interlayer insulating film 136. FIGS. 28 and 29 also show a substrate 122 provided to face the active matrix substrate with a liquid crystal layer 112 interposed therebetween.
In the active matrix substrate with the above configuration, the interlayer insulating film 136 is formed between the gate signal line 104 or the source signal line 102 and the pixel electrode 140. This allows the periphery of the pixel electrode 140 to overlap the signal lines 102 and 104. As a result, a liquid crystal display device with a high aperture ratio can be obtained. Moreover, the overlapping pixel electrode 140 shields an electric field generated due to the potential at the signal lines, effectively suppressing failure in the orientation of liquid crystal molecules.
Referring to FIGS. 28 and 29, a light-shading layer 144, and color layers 146 exhibiting red, blue, or green constituting a color filter are formed on the substrate 122 facing the active matrix substrate with the liquid crystal layer 112 therebetween. A counter electrode 148 and an alignment film 150 are formed in this order on the color filter. Another alignment film 150 is formed on the surface of the active matrix substrate in contact with the liquid crystal layer 112.
FIG. 30A is an enlarged plan view of a portion of FIG. 27 where the gate signal line 104 and the source signal line 102 cross each other. FIG. 30B is a sectional view taken along line 30Bxe2x80x9430B of FIG. 30A, showing the overlap portions of the pixel electrodes 140 on the source signal line 102.
Referring to FIG. 30A, the vertically adjacent pixel electrodes 140 overlap the corresponding gate signal line 104 by overlap widths dg1 and dg2, while the horizontally adjacent pixel electrodes 140 overlap the corresponding source signal line 102 by overlap widths ds1 and ds2. These overlap widths are generally determined in consideration of the processing precision of the gate signal lines 104 and the source signal lines 102 which serve as light-shading films, the overlap precision of the pixel electrodes 140 on the gate signal lines 104 and the source signal lines 102, and the processing precision of the pixel electrodes 140. Conventionally, the pixel electrodes 140 overlap the gate signal lines 104 and the source signal lines 102 so that the overlap widths dg1 and dg2 are equal to each other and the overlap widths ds1 and ds2 are equal to each other.
The liquid crystal display device where the pixel electrodes overlap the signal lines as described above causes no problem as far as it is driven by a flame inversion driving method. However, when such a liquid crystal display device is driven by a gate line inversion driving method, a source line inversion driving method, or a dot inversion driving method, the following problem arises. That is, referring now to FIG. 30B, the orientation of liquid crystal molecules 152a is disturbed due to an electric field generated between the adjacent pixel electrodes, generating a reverse tilt domain having liquid crystal molecules 152b which have a reverse pretilt angle, i.e., are oriented in the opposite direction of an orientation direction D1 (see FIG. 30A) of the liquid crystal molecules 152a. The generation of such a reverse tilt domain causes light leakage and thus eminently degrades the display characteristics of the resultant liquid crystal display device.
In order to prevent light leakage of the liquid crystal display device due to the disturbance of the orientation of liquid crystal molecules, increasing the overlap widths of the pixel electrodes on the gate signal lines and the source signal lines is known. Increasing the overlap widths, however, causes another problem of increasing the occupation of the light-shading portions in the liquid crystal display device and thus decreasing the aperture ratio.
Also known is a liquid crystal display device where each pixel is divided into two portions having different orientation directions D2 and D2 of liquid crystal molecules as shown in FIGS. 31A to 31C. FIG. 31A is a plan view of a portion of such a liquid crystal display device where a gate signal line 104 and a source signal line 102 cross each other. FIG. 31B is a sectional view taken along line 31Bxe2x80x9431B of FIG. 31A, and FIG. 31C is a sectional view taken along line 31Cxe2x80x9431C of FIG. 31A.
In such a liquid crystal display device, also, pixel electrodes conventionally overlap signal lines so that overlap widths dg1 and dg2 are equal to each other and overlap widths. ds1 and ds2 are equal to each other as shown in FIG. 31A. This causes no problem as far as the liquid crystal display device is driven by a flame inversion driving method. However, as in the above case, when it is driven by a gate line inversion driving method, a source line inversion driving method, or a dot inversion driving method, the following problem arises. That is, the orientation of liquid crystal molecules 152a are disturbed due to an electric field generated between the adjacent pixel electrodes, generating a reverse tilt domain having liquid crystal molecules 152b which have a reverse pretilt angle as shown in FIGS. 31B and 31C. This causes light leakage and thus eminently degrades the display characteristics of the resultant liquid crystal display device.
In this case, as in the above case, the overlap widths of the pixel electrodes on the gate signal lines and the source signal lines may be increased to prevent light leakage due to the disturbance of the orientation of liquid crystal molecules. However, this causes another problem of increasing the occupation of the light-shading portions in the liquid crystal display device and thus decreasing the aperture ratio.
Referring to FIGS. 27 and 28 again, a reverse tilt domain is also generated in a region at and around each contact hole 142 as indicated by the reference numeral 154. Such a reverse tilt domain tends to be generated especially when the angle of the inner wall of the contact hole 142 with respect to the substrate surface exceeds 45xc2x0. Light leakage may occur even when the liquid crystal layer 112 is switched from a light-transmitting state to a light-shading state.
In order to prevent light leakage at and around the contact hole, it is known to shade the region at and around the contact hole 142 by using a light-shading material for the storage capacitor electrode 126 above which the contact hole 142 is formed. For complete light-shading, however, the size of the storage capacitor electrode 126 needs to be sufficiently large. This causes a problem of substantially decreasing the display area of each pixel and thus decreasing the aperture ratio of the resultant liquid crystal display device.
Japanese Laid-open Patent Publication No. 5-249494 discloses a method for suppressing the generation of a reverse tilt domain in an active matrix liquid crystal display device. In the disclosed method, the angles of sloped steps between pixel electrodes and gate and source signal lines with respect to a substrate surface are set at 60xc2x0 or less, thereby preventing a generation of disclination lines on a display screen.
According to the above method, however, as described in the Publication, a satisfactory result is not obtainable when the difference between the pixel electrodes and the gate and signal lines exceeds 2 xcexcm. As for the contact hole 142 shown in FIG. 28, for example, a reverse tilt domain is generated at and around the contact hole 142 when the difference (corresponding to the thickness of the interlayer insulating film 136 ) exceeds 2 xcexcm. Therefore, in order to apply the disclosed method to the contact hole 142, the thickness of the interlayer insulating film 136 should be 2 xcexcm or less.
The interlayer insulating film 136, on the other hand, needs to be sufficiently thick to have a flat surface, which is required to flatten the alignment film 150 to be formed above the interlayer insulating film 136 in contact with the liquid crystal layer 112. Accordingly, it is substantially difficult to set the thickness of the interlayer insulating film 136 at 2 xcexcm or less. The above disclosed method is therefore not applicable to the region including the contact hole.
The liquid crystal display device of this invention includes: gate signal lines; source signal lines crossing the gate signal lines; an interlayer insulating film formed on the gate signal lines and the source signal lines; and pixel electrodes formed on the interlayer insulating film, wherein a first pixel electrode and a second pixel electrode adjacent to each other at both sides of the gate signal line partially overlap the gate signal line sandwiched by the first pixel electrode and the second pixel electrode, and an overlap width of the first pixel electrode on the gate signal line is different from an overlap width of the second pixel electrode on the gate signal line.
In one embodiment of the invention, the first pixel electrode is located downstream of a direction of a pretilt angle of liquid crystal molecules with respect to the gate-signal line and the second pixel electrode is located upstream of the direction of the pretilt angle of liquid crystal molecules, and the overlap width of the first pixel electrode on the gate signal line is larger than the overlap width of the second pixel electrode on the gate signal line.
In another embodiment of the invention, the liquid crystal display device is driven by a gate line inversion driving method.
Alternatively, the liquid crystal display device of this invention includes: gate signal lines; source signal lines crossing the gate signal lines; an interlayer insulating film formed on the gate signal lines and the source signal lines; and pixel electrodes formed on the interlayer insulating film, wherein a third pixel electrode and a fourth pixel electrode adjacent to each other at both sides of the source signal line partially overlap the source signal line sandwiched by the third pixel electrode and the fourth pixel electrode, and an overlap width of the third pixel electrode on the source signal line is different from an overlap width of the fourth pixel electrode on the source signal line.
In one embodiment of the invention, the third pixel electrode is located downstream of a direction of a pretilt angle of liquid crystal molecules with respect to the source signal line and the fourth pixel electrode is located upstream of the direction of the pretilt angle of liquid crystal molecules, and the overlap width of the third pixel electrode on the source signal line is larger than the overlap width of the fourth pixel electrode on the source signal line.
In another embodiment of the invention, the liquid crystal display device is driven by a source line inversion driving method.
Alternatively, the liquid crystal display device of this invention includes: gate signal lines; source signal lines crossing the gate signal lines; an interlayer insulating film formed on the gate signal lines and the source signal lines; and pixel electrodes formed on the interlayer insulating film, wherein a first pixel electrode and a second pixel electrode adjacent to each other at both sides of the gate signal lines partially overlap the gate signal line sandwiched by the first pixel electrode and the second pixel electrode, an overlap width of the first pixel electrode on the gate signal line is different from an overlap width of the second pixel electrode on the gate signal line, a third pixel electrode and a fourth pixel electrode adjacent to each other at both sides of the source signal lines partially overlap the source signal line sandwiched by the third pixel electrode and the fourth pixel electrode, and an overlap width of the third pixel electrode on the source signal line is different from an overlap width of the fourth pixel electrode on the source signal line.
In one embodiment of the invention, the first pixel electrode is located downstream of a direction of a pretilt angle of liquid crystal molecules with respect to the gate signal line and the second pixel electrode is located upstream of the direction of the pretilt angle of liquid crystal molecules, and the overlap width of the first pixel electrode on the gate signal line is larger than the overlap width of the second pixel electrode on the gate signal line, and the third pixel electrode is located downstream of a direction of a pretilt angle of liquid crystal molecules with respect to the source signal line and the fourth pixel electrode is located upstream of the direction of the pretilt angle of liquid crystal molecules, and the overlap width of the third pixel electrode on the source signal line is larger than the overlap width of the fourth pixel electrode on the source signal line.
In another embodiment of the invention, the liquid crystal display device is driven by a dot inversion driving method.
Alternatively, the liquid crystal display device of this invention includes: gate signal lines; source signal lines crossing the gate signal lines; an interlayer insulating film formed on the gate signal lines and the source signal lines; and pixel electrodes formed on the interlayer insulating film, wherein each of the pixel electrodes has a first region and a second region which are adjacent to each other and have different orientation directions of liquid crystal molecules, the first region and the second region of each pixel electrode partially overlap at least one signal line of the gate signal line and the source signal line, an overlap width of the first region on the signal line is different from an overlap width of the second region on the signal line, and a boundary of the first region and the second region is covered with a light-shading film which crosses the signal line.
In one embodiment of the invention, the signal line is the source signal line, the source signal line is located downstream of a direction of a pretilt angle of liquid crystal molecules in the first region, the source signal line is located upstream of the direction of the pretilt angle of liquid crystal molecules in the second region, and the overlap width of the second region on the source signal line is larger than the overlap width of the first region on the source signal line.
In another embodiment of the invention, the signal line is substantially linear, and an edge of the first region of the pixel electrode overlapping the signal line is offset from an edge of the second region overlapping the signal line.
In still another embodiment of the invention, an end of a portion of the signal line which is overlapped by the first region is offset from an end of a portion of the signal line which is overlapped by the second region, and an edge of the first region overlapping the signal line is aligned with an edge of the second region over-lapping the signal line.
In still another embodiment of the invention, the liquid crystal-display device is driven by a source line inversion driving method or a dot inversion driving method.
Alternatively, the liquid crystal display device of this invention includes: gate signal lines; source signal lines crossing the gate signal lines; an interlayer insulating film formed on the gate signal lines and the source signal lines; pixel electrodes formed on the interlayer insulating film; and drain electrodes connected to the corresponding pixel electrodes via contact holes formed through the interlayer insulating film, wherein each of the contact holes is formed above a light-shading signal line located under the corresponding drain electrode, and a center axis of the contact hole is offset from a center axis of the light-shading signal line.
In one embodiment of the invention, the center axis of the contact hole is offset from the center axis of the light-shading signal line in a direction of a pretilt angle of liquid crystal molecules by a distance.
In another embodiment of the invention, the distance between the center axis of the contact hole and the center axis of the light-shading signal line is in a range of about 0.5 to about 1.5 xcexcm.
In still another embodiment of the invention, a gate electrode of each of switching elements is disposed in the center of the corresponding pixel electrode, and the light-shading signal line is disposed between the pixel electrode and a pixel electrode adjacent in a direction opposite to a direction of a pretilt angle of liquid crystal molecules.
In still another embodiment of the invention, the light-shading signal line constitutes a gate electrode of each of the switching elements, and the gate electrode is disposed between the pixel electrode and a pixel electrode adjacent in a direction opposite to a direction of a pretilt angle of liquid crystal molecules.
Thus, according to one embodiment of the liquid crystal display device of the present invention, the overlap width of the first pixel electrode on the gate signal line is different from that of the second pixel electrode on the gate signal line. Typically, it is designed that the overlap width of the first pixel electrode on the gate signal line is larger than the overlap width of the second pixel electrode on the gate signal line. In a liquid crystal display device employing the gate line inversion driving method, a reverse tilt domain tends to be generated in the overlap portion of the first pixel electrode of the two pixel electrodes sandwiching each gate signal line which is located downstream of the direction of the pretilt angle (orientation direction) of liquid crystal molecules. In the liquid crystal display device of this embodiment, a reverse tilt domain generated due to an electric field generated between adjacent pixel electrodes can be covered with the overlap portion of the first pixel electrode on the gate signal line with a large overlap width. Thus, the liquid crystal display device of this embodiment can prevent light leakage due to the generation of a reverse tilt domain when it is driven by the gate line inversion method, while it holds a high aperture ratio.
According to another embodiment of the liquid crystal display device of the present invention, the overlap width of the third pixel electrode on the source signal line is different from that of the fourth pixel electrode on the source signal line. Typically, it is designed that the overlap width of the third pixel electrode on the source signal line is larger than the overlap width of the fourth pixel electrode on the source signal line. In a liquid crystal display device employing the source line inversion driving method, a reverse tilt domain tends to be generated in the overlap portion of the third pixel electrode of the two pixel electrodes sandwiching each source signal line which is located downstream of the direction of the pretilt angle (the orientation direction) of liquid crystal molecules. In the liquid crystal display device of this embodiment, a reverse tilt domain generated due to an electric field generated between adjacent pixel electrodes can be covered with the overlap portion of the third pixel electrode on the source signal line with a large overlap width. Thus, the liquid crystal display device of this embodiment can prevent light leakage due to the generation of a reverse tilt domain when it is driven by the source line inversion method, while it holds a high aperture ratio.
According to still another embodiment of the liquid crystal display device of the present invention, the overlap width of the first pixel electrode on the gate signal line is different from that of the second pixel electrode on the gate signal line, and the overlap width of the third pixel electrode on the source signal line is different from that of the fourth pixel electrode on the source signal line. Typically, it is designed that the overlap width of the first pixel electrode on the gate signal line is larger than the overlap width of the second pixel electrode on the gate signal line and that the overlap width of the third pixel electrode on the source signal line is larger than the overlap width of the fourth pixel electrode on the source signal line. In a liquid crystal display device employing the dot inversion driving method, reverse tilt domains tend to be generated in the overlap portion of the first pixel electrode of the two pixel electrodes sandwiching each gate signal line which is located downstream of the direction of the pretilt angle (orientation direction) of liquid crystal molecules, and the overlap portion of the third pixel electrode of the two pixel electrodes sandwiching each source signal line which is located downstream of the direction of the pretilt angle (orientation direction) of liquid crystal molecules. In the liquid crystal display device of this embodiment, reverse tilt domains generated due to an electric field generated between adjacent pixel electrodes can be covered with the overlap portion of the first pixel electrode on the gate signal line with a large overlap width and the overlap portion of the third pixel electrode on the source signal line with a large overlap width. Thus, the liquid crystal display device of this embodiment can prevent light leakage due to the generation of reverse tilt domains when it is driven by the dot inversion method, while it holds a high aperture ratio.
According to still another embodiment of the liquid crystal display device of the present invention, each pixel electrode has adjacent first and second regions having different orientation directions of liquid crystal molecules. The overlap width of the first region on a signal line is different from that of the second region on the signal line. The boundary of the first and second regions is covered with a light-shading film which is formed to cross the signal line. For example, assume that the first and second regions of the pixel electrode overlap a source signal line, and the source signal line is located downstream of the direction of the pretilt angle (orientation direction) of liquid crystal molecules in the first region with respect to the first region, while the source signal line is located upstream of the direction of the pretilt angle of liquid crystal molecules in the second region with respect to the second region. In this case, a reverse tilt domain is generated in the second region on the side of the source signal line. According to the present invention, it is typically designed that the overlap width of the second region on the source signal line is larger than the overlap width of the first region on the source signal line. With this arrangement, a reverse tilt domain generated in the overlap portion on the source signal line can be well shaded from light by the wide overlap portion. Also, since the boundary of the first and second regions is covered with a light-shading film, light leakage from the boundary can be prevented.
According to still another embodiment of the liquid crystal display device of the present invention, each contact hole is covered with a light-shading signal line underlying a drain electrode, and the center axis of the contact hole does not correspond with the center axis of the light-shading signal line. Typically, the center axis of each contact hole is offset from the center axis of the light-shading signal line in the direction of the pretilt angle of liquid crystal molecules. Accordingly, a reverse tilt domain generated at and around the contact hole is completely covered with the light-shading signal line. Thus, the liquid crystal display device of this embodiment can prevent light leakage due to the generation of a reverse tilt domain while it holds a high aperture ratio.
Thus, the invention described herein makes possible the advantages of: (1) providing a liquid crystal display device with a high aperture ratio which can prevent light leakage caused by the generation of a reverse tilt domain between a gate signal line and a pixel electrode and/or between a source signal line and the pixel electrode; (2) providing a liquid crystal display device with a high aperture ratio which can prevent light leakage caused by the generation of a reverse tilt domain even if any of the gate line inversion driving method, the source line inversion driving method, and the dot inversion driving method are employed; and (3) providing a liquid crystal display device with a high aperture ratio which can prevent light leakage caused by the generation of a reverse tilt domain at and around a contact hole irrespective of the thickness of the interlayer insulating film.
These and other advantages of the present invention will become apparent to those skilled in the art upon reading and understanding the following detailed description with reference to the accompanying figures.